The Waterfall Model was first Process Model to be introduced. It is also referred to as a linear-sequential life cycle model.  It is very simple to understand and use.  In a waterfall model, each phase must be completed fully before the next phase can begin. This type of software development model is basically used for the project which is small and there are no uncertain requirements.

At the end of each phase, a review takes place to determine if the project is on the right path and whether or not to continue or discard the project.

In this model software testing starts only after the development is complete. In waterfall model phases do not overlap.

Diagram of Waterfall model

Phases of Waterfall Model in Software Engineering

There are several phases in the waterfall model. They are briefly explained below. Let us understand the concept of Waterfall model with example of a banking application for illustrating the topic.

Let us assume that the Citibank is planning to have a new banking application developed and they have approached your organization in the 1990’s.

Requirements Gathering and Analysis

In this phase the requirements are gathered by the business analyst and they are analyzed by the team. Requirements are documented during this phase and clarifications can be sought.

The Business Analysts document the requirement based on their discussion with the customer.

Going through the requirements and analyzing them has revealed that the project team needs answers to the following questions which were not covered in the requirements document –

• Will the new banking application be used in more than one country?
• Do we have to support multiple languages?
• How many users are expected to use the application? etc

System Design

The architect and senior members of the team work on the software architecture, high level and low level design for the project.

It is decided that the banking application needs to have redundant backup and failover capabilities such that system is accessible at all times.

The architect creates the Architecture diagrams and high level / low level design documents.

Implementation

The development team works on coding the project.

They take the design documents / artifacts and ensure that their solution follows the design finalized by the architect.

Since the application is a banking application and security was a high priority in the application requirements, they implement several security checks, audit logging features in the application.

They also perform several other activities like a senior developer reviewing the other developers code for any issues. Some developers perform static analysis of the code.

Testing

The testing team tests the complete application and identifies any defects in the application.

These defects are fixed by the developers and the testing team tests the fixes to ensure that the defect is fixed.

They also perform regression testing of the application to see if any new defects were introduced.

Testers with banking domain knowledge were also hired for the project so that they could test the application based on the domain perspective.

Security testing teams were assigned to test the security of the banking application.

Deployment

The team builds and installs the application on the servers which were procured for the banking application.

Some of the high level activities include installing the OS on the servers, installing security patches, hardening the servers, installing web servers and application servers, installing the database etc.

They also co-ordinate with network and IT administrative teams etc to finally get the application up and running on the production servers.

Maintenance

During the maintenance phase, the team ensures that the application is running smoothly on the servers without any downtime.

Issues that are reported after going live are fixed by the team and tested by the testing team.

Examples of Waterfall Model

In the olden days, Waterfall model was used to develop enterprise applications like Customer Relationship Management (CRM) systems, Human Resource Management Systems (HRMS), Supply Chain Management Systems, Inventory Management Systems, Point of Sales (POS) systems for Retail chains etc.

Waterfall model was used significantly in the development of software till the year 2000. Even after the Agile manifesto was published in 2001, Waterfall model continued to be used by many organization till the last decade.

These days most project follow Agile Methodology, some form of Iterative model or one of the other models depending on their project specific requirement.

In the olden days, applications developed in Waterfall Model like CRM Systems, Supply Chain Management Systems etc would usually take a year or longer to develop.

With the evolution of technology, there were cases where large scale enterprise systems were developed over a period of 2 to 3 years but were redundant by the time they were completed. There were several reasons for this.

• By the time the applications were developed in C, C++ etc, new languages (relatively speaking) like Java, .Net etc would replace them with web based functionality.
• Even if the application was developed using a new technology, factors like more competitors entering the market, cheaper alternatives becoming available, better functionality using newer technologies, change in customers requirement etc. increase the risk of developing an application over several years.

However, there are some areas where Waterfall model was continued to be preferred.

• Consider a system where human life is on the line, where a system failure could result in one or more deaths.
• In some countries, such mishaps could lead to imprisonment for those who are accountable.
• Consider a system where time and money were secondary considerations and human safety was first.

In such situations, Waterfall model was the preferred approach.

• Development of Department Of Defense (DOD), military and aircraft programs followed Waterfall model in many organizations.
• This is because of the strict standards and requirements that have to be followed.
• In such industries, the requirements are known well in advance and contracts are very specific about the deliverable of the project.
• DOD Agencies typically considered Waterfall model to be compatible with their acquisition process and rigorous oversight process required by the government.

Having said that, even these industries are being disrupted by the use of Iterative model and Agile methodology by organizations like Space X and others.

Waterfall model was also used in banking, healthcare, control system for nuclear facilities, space shuttles etc

Advantages of waterfall model

• This model is simple and easy to understand and use.
• It is easy to manage due to the rigidity of the model – each phase has specific deliverables and a review process.
• In this model phases are processed and completed one at a time. Phases do not overlap.
• Waterfall model works well for smaller projects where requirements are clearly defined and very well understood.

Disadvantages of waterfall model

• Once an application is in the testing stage, it is very difficult to go back and change something that was not well-thought out in the concept stage.
• No working software is produced until late during the life cycle.
• High amounts of risk and uncertainty.
• Not a good model for complex and object-oriented projects.
• Poor model for long and ongoing projects.
• Not suitable for the projects where requirements are at a moderate to high risk of changing.

When to use the waterfall model

• This model is used only when the requirements are very well known, clear and fixed.
• Product definition is stable.
• Technology is understood.
• There are no ambiguous requirements
• Ample resources with required expertise are available freely
• The project is short.

In Waterfall model, very less customer interaction is involved during the development of the product. Once the product is ready then only it can be demonstrated to the end users.

Once the product is developed and if any failure occurs then the cost of fixing such issues are very high, because we need to update everything from document till the logic.

In today’s world, Waterfall model has been replaced by other models like iterative, agile etc.

Difference between: Waterfall Model vs Agile Model

There are many differences between Waterfall and Agile model as shown below.

Waterfall Model	Agile Model
Planning – Waterfall model requires planning for long term which requires complete clarity in requirements	Planning in Agile projects is generally on a short term since the work product is delivered in 2 to 4 weeks
Project success is dependent on implementing the requirements closely	Project success is based on delivery of business value to the client
Waterfall projects are driven from the top of a projects hierarchy	Teams are self governing in an Agile project
Sequential planning with clearly defined milestone details and predictability are characteristics of Waterfall projects	Planning in Agile projects is iterative in nature and adapts to changing requirements
There are many roles in a Waterfall project and these projects can also have several levels of hierarchy	Agile projects teams have significantly lesser roles, for example Scrum teams can get by with only three roles
There is significant amount of communication with the user during requirements gathering (at the beginning) and testing (at the end)	There is a steady on-going communication with the user during the project
There is no much dependency on the end users during the development and other intermediate phases	There is significant dependency on the users, all the way through the lifetime of the project
Complete requirements are clearly documented to begin with	Requirements continue to develop over the lifetime of the project and are defined when they are needed, i.e, just in time
Team members with different roles have different responsibility levels	Equal responsibility is shared between the roles
Change control is strictly enforced and rigorous change control processes are followed. Thus change is discouraged and they do not respond well to change.	Agile projects are open to change, they accept them openly and respond well
Quality Control activities like Testing are performed towards the end of the project	Quality Control activities are performed throughout the project
The steps in the processes in Waterfall model are rigorously followed since this model is more process oriented	Agile model is more people oriented and lesser importance is placed on processes with the option to skip those processes whose value is low
Delivery of the project at the end is characterized by a big bang event	Working product features are delivered in each sprint of the project
It is difficult to measure the progress of the project in the middle of the project	Progress of the project can be easily measured since working features are delivered frequently
Progress of the project is generally reviewed with the team once a week	Progress of the project is reviewed with the team on a daily basis in the standup meeting

Have you seen Waterfall model being used in your organization? Please share your experience in the comments below.

Also see:

What is Agile Model?
What is V-model?
What is Incremental model?
What is Spiral model?
What is Prototype model?
What is RAD model?

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